The present invention relates to an electrostatic capacitance detection device that reads the surface shape of an object having microscopic irregularities such as fingerprints by detecting an electrostatic capacitance that varies in response to the distance to the surface of the object.
In the electrostatic capacitance detection device used for a fingerprint sensor or the like, a dielectric film is prepared on an electrostatic capacitance detection electrode, and fingerprints are detected utilizing the fact that there will be produced differences in the electrostatic capacitance between the ridges and valleys of the fingerprint when a finger touches thereupon. Japanese Unexamined Patent Publication No. 2003-254706 is an example of the related art, in which an electrostatic capacitance detection device with a circuit configuration is disclosed.
This electrostatic capacitance detection device of the conventional art includes: M individual power supply lines and N individual output lines arranged in a matrix form of M rows by N columns; and electrostatic capacitance detection elements prepared in these intersections, wherein the electrostatic capacitance detection element includes a signal detection element and a signal amplifying element; the signal detection element includes a capacitance detection electrode and a capacitance detection dielectric film; and wherein the signal amplifying element comprises an MIS thin film transistor used for signal amplification comprised of a gate electrode, a gate insulating film, and a semiconductor film.
However, in the above-described circuit configuration, there is a problem that signals from a plurality of electrostatic capacitance detection elements may be mixed in some cases, thereby reducing the detection accuracy. This will be described hereinafter.
FIG. 9 shows a circuit diagram of the electrostatic capacitance detection device shown in the above-described patent. FIG. 9 shows a case where a voltage ΔV is applied to between a power supply line PLi and an output line OLj, thereby taking out a signal OSij from an electrostatic capacitance detection element ECSEij to the output line OLj. The flow of the signal OSij which is originally intended to be outputted is the current path A shown by the thick solid line from the power supply line PLi to the output line OLj. However, in the state that the voltage ΔV is applied to between the power supply line PLi and the output line OLj, there are a plurality of paths shown by the dotted lines in the view such as the current path B from an electrostatic capacitance detection element ECSEi(j+1) to ECSE(i+1)(j+1) to ECSE(i+1)j, the current path C from an electrostatic capacitance detection element ECSEi(j+1) to ECSE(i+2)(j+1) to ECSE(i+2)j, or the like in addition to the current path A. Because of the presence of such current paths, unless an adequate threshold can be set to the electrostatic capacitance detection element, the signals will be mixed and outputted to the output line OLj via the current paths B and C, thereby reducing the detection accuracy.